Headboxes are included in the wet end of a paper machine and are used to distribute the stock evenly across the width of the wire and to control the discharge so that this takes place at a uniform velocity and in the same direction across the entire width of the wire. With increasingly higher paper machine speeds, there has been a changeover to closed headboxes of the kind described above which operate under pressure. At the same time the size of the headboxes has been able to be limited. While the industry has demanded higher production rates for the paper machines, it also requires that the same high quality standards be maintained. For special headboxes for two or more stock jets, i.e. so-called multilayer headboxes, the higher production speeds involve major problems in producing a paper web of satisfactory quality with regard to desired uniformity of basis weight and thickness of the combined layers. It is therefore of great importance that the desired pressure can be maintained in the headbox and that the heights of the channels, i.e. the distances between two channel-forming surfaces, be kept constant, so that the discharged stock jets will be uniform with respect to velocity and thickness. Multilayer headboxes of this kind are disclosed, for example, in U.S. Pat. No. 3,598,696 (Beck); U.S. Pat. No. 3,839,143 (Suckow); U.S. Pat. No. 3,923,593 (Verseput); U.S. Pat. No. 4,021,295 (Schmaeng) and U.S. Pat. No. 4,086,130 (Justus).
It is known from the aforementioned U.S. Pat. No. 4,086,130 to automatically maintain control of the stock flows passing through the stock channels in the headbox that are intended for the outer layers dependent on the stock flow passing through the stock channel intended for the center layer, in that actual values of the pressure drop between two points located upstream and downstream of a transverse perforated plate or similar perforated partition in the center channel of the headbox are recorded and processed in order to be compared with operational setpoints. It is also known through this patent to control with a controller means the total hydraulic head, so that the desired spouting velocity of the stocks is obtained in relation to the speed of the paper machine wire. This known system is unsatisfactory, however, in that no actual control of the layer thickness is obtained and that if a blockage should occur in one or more tubes or the like in the portion of the headbox aligning the stock flow for the channel in question, this would cause the pressure drop across this portion to increase and the flow through the same to diminish. This increase of pressure drop would be interpreted by the control system as if the flow through the channel was too large and it would therefore reduce the flow still more, an action that is exactly the opposite of that required and one that would also cause the jet velocity from the headbox to be reduced. At a change of the ratio between pressures in the converging portions of the channels, the partitions will be affected so that, if they are self-adjusting dependent on the pressure difference across them, the heights of the channels will be changed and the desired relationship between layer thicknesses can no longer be obtained.